Service Suggestions/Engine, Transmission etc

Hi guys, I am new to community. Need your experience guys.

I bought my first car, used 2005 Subaru Impreza 2.5RS (stock). I already changed transmission and rear differential oil, which gave me much much better drive. I am also going to flush engine and do first oil/oil filter change tomorrow [first time since I bought it (oil change is due after 3K km)]. Engine seems tuned. And oil still looks great, should I change it anyway? My question is what is best synthetic oil to use and is there any additives I should use? Guy before me was using Castrol syntec.

I also used Iso-Heet to remove water from fuel line - great boost in performance, and Lucas Oil Engine cleaner - not much difference noticed. Q is it okay to use these products regularly? Or just use them once every 3 months or so?..

I also thinking of installing short ram intake - which would you recommend? Or should I just install K&N air filter on stock air intake?

Sway bar?

Struts bar?

And maybe 17"-18" tires.

I have changed front breaks and rotors, rear were recently installed.

Body, inside where the driver seat fixed to the floor I have quite a bit of rust now (this is new we had few snow storms, and I was being not accurate got lots of snow on my front driver seat/area a couple of times >.<) Should I scrape that rust and apply something? Or just leave it?

Any suggestions, recommendations guys.

Thanks!